A variable focal length lens can include a gradient index (GRIN) lens, whose focal length can be controlled electrostatically, or a flexible polymeric lens whose focal length can be controlled mechanically. Both technologies have inherent limitations that impose restrictions on the performance of these existing tunable lenses.
Gradient index lenses have inherent limitations associated with the relatively small electro-optic coefficients found in the majority of electro-optic materials. This results in a small optical path modulation and, therefore, requires thick lenses or very high voltages to be employed. In addition, many electro-optic materials show a strong birefringence that causes polarization dependence of the lens properties.
Mechanically adjustable flexible lenses typically have a substantially wider range of tunability than GRIN lenses. However, they require external actuation devices, such as micropumps, to operate. Integration of such devices involves substantial problems, especially severe in the case where a two-dimensional array of tunable lenses is required.